IL-4 is a pleiotropic cytokine that plays a central role in immunoregulation. Although both T and mast cells express IL-4, each cell type utilizes distinct cis- and trans-acting factors to regulate transcription. A number of both promoter-proximal and intronic DNA elements are targets of lineage-specific factors. These factors include proteins belonging to the NF-AT, STAT, GATA, Ets, and c-maf families. Such differences likely reflect the distinct extracellular signals that elicit IL-4 in these cell types and the unique role each cell type has in the immune response. In addition to transcriptional activation of the IL-4 gene in differentiated cells, there is a clearly an additional level of regulation. Recent studies of T cells as well as those in mast cells by Dr. Brown's laboratory have established a role for developmentally regulated chromatin accessibility in IL-4-gene expression. DNAseI hypersensitivity and demethylation within the IL-4 chromosomal gene locus are observed at an early stage in the commitment to a Th2 or mast cell lineage. Importantly, these hallmarks of "open" chromatin evident in both cell types are independent of active IL-4 transcription. Dr. Brown proposes that these observations demonstrate that two temporally distinct events regulate the acquisition of an IL-4-producing phenotype: 1) developmentally-determined signals confer locus opening and 2) in differentiated cells, cell activation signals promote the association of transcription factors with accessible regulatory elements within the IL-4 gene. The studies described in this proposal will continue Dr. Brown's ongoing investigation on the cell-type specific regulatory elements that dictate IL-4 gene expression in T and mast cells. Her recent data suggests that some of these regulatory elements have striking parallels to the u and k Ig gene intron enhancers. Although initially defined as regulatory elements based on their ability to enhance transcription in conventional reporter assays, Eu and Ek also function to regulate lineage and cell-stage specific accessibility of the Ig locus. Dr. Brown hypothesizes that 1) the IL-4 gene is regulated in a similar manner; some sequences defined as enhancers have dual roles as accessibility determinants; 2) lineage specific factors regulate chromatin accessibility at the IL-4 gene locus in T and mast cells; and 3) differences in the general locus accessibility of the IL-4/IL-13 gene cluster contribute to strain-specific differences in IL-4 gene expression levels. The specific aims of this proposal are: 1. To determine the mechanisms that underlie the ability of the intron "enhancer" to mediate chromatin accessibility in mast cells and determine its potential role in T cells. 2. To characterize the cis- and trans-acting elements that comprise a defined enhancer located in the IL-4/IL-13 intergenic region and determine its in vivo role. 3. To identify other sequences that influence transcription of IL-4 and/or IL-13 in both T and mast cells. 4. To examine the molecular basis of strain-specific differences in murine IL-4 gene expression.